Large area plasma source

ABSTRACT

A chamber housing ( 2 ) enclosing a plasma region ( 20 ) in a large area plasma source used for performing plasma assisted processes in large area substrates, the chamber housing ( 2 ) being composed of: a housing member ( 2 ) constituting a substantially vertically extending wall ( 4 ) surrounding a space ( 6 ) corresponding to the plasma region ( 10 ), the housing member ( 2 ) having a plurality of openings ( 32 ) and electrically conducive elements forming an electrostatic shield around the space; a plurality of dielectric members ( 36 ) each having a peripheral edge and each disposed to close a respective opening ( 23 ); and sealing members ( 40, 40′, 42, 42′ ) forming a hermetic seal between said housing member and said peripheral edge of each of said dielectric members ( 36 ).

This application is the National Phase of International ApplicationPCT/US99/27928 filed Dec. 10, 1999 which designated the U.S. and thatInternational Application was published under PCT Article 21(2) inEnglish. This application also claims priority from U.S. provisionalapplication No. 60/114,454 filed on Dec. 30, 1998.

BACKGROUND OF THE INVENTION

The present invention relates to plasma sources for use in theperformance of plasma-assisted processes, including deposition andetching processes performed on substrates in processing chambers. Theinvention particularly relates to plasma sources which allow processingof large area substrates.

There is a demand for plasma sources that will enable processes of theabove-mentioned type to be performed on large size wafers and even moreso for flat panel display processing. There are indications in theindustry that efforts will be made to manufacture flat panel displaysmeasuring 1 meter on a side and plasma-assisted processing of suchsubstrates will require higher plasma ion density levels than areproduced in existing systems. Plasma-assisted processing of such largearea substrates requires both high plasma density and high pumping speedto achieve high processing rates.

In plasma sources of the type described above, the plasma deposition oretching rate will depend on the ion flux, or ion density, as long as theprocess gas throughput, or pumping speed, satisfies the processingchamber requirements. Therefore, the achievement of satisfactoryprocessing rates for large area substrates requires both the gasthroughput and the ion flux be sufficiently high.

In addition, a plasma source having the requisite large dimensions mustwithstand a considerable force from atmospheric pressure and must becapable of providing an optimum geometry for creation of an electricfield that will provide a uniform plasma inside the processing chamberof the source.

BRIEF SUMMARY OF THE INVENTION

It is an object of the present invention to provide a large area plasmasource which has the above-mentioned capabilities.

Another object of the invention is to provide a large area plasma sourcehousing capable of supporting atmospheric pressure forces whileproviding a requisite electrostatic shield for the plasma confinedwithin the housing and permitting transmission of RF electromagneticfield energy to the plasma.

The invention achieves these and other objects by providing a plasmasource housing having side walls made of: metal electrostatic shieldmembers that provide support against atmospheric pressure; or a ridgeddielectric wall that is capable of supporting atmospheric pressure andis combined with electrically conductive elements that provide theelectrostatic shield function; or a combination of the two. These wallscan be shaped according to any vertical geometry including, but notlimited to, straight, tapering in or out, curved in or out, etc.Therefore, a plasma source housing can be constructed to have virtuallyany dimensions and shape needed, while allowing RF energy to be suppliedto the plasma through the housing wall. In addition, this housing willreadily accommodate a system for cooling the processing chamber walls.

A further object of the invention is to achieve a high degree of plasmauniformity within the processing chamber. Because the RF electric fieldwhich creates and maintains the plasma originates in the region whichsurrounds the processing chamber, plasma uniformity is attained bydiffusion, with gas species, or processing gas, flow and plasma gradientcombining to provide process uniformity. Therefore, at any pressure andRF power level, plasma uniformity is a function of the aspect ratio ofthe processing chamber, i.e., the ratio of the square root of the crosssectional area to the height of the processing chamber. Thecross-sectional area is the area of a horizontal plane at a locationwhere the chamber has an average cross-sectional area.

It presently appears that by applying the principles to be disclosedherein together with standard testing procedures, a high degree ofplasma uniformity can be achieved.

The above and other objects are achieved, according to the presentinvention, by a chamber housing enclosing a plasma region in a largearea plasma source used for performing plasma assisted processes onlarge area substrates, the chamber housing comprising:

a housing member constituting a substantially vertically extending wallsurrounding a space corresponding to the plasma region, the housingmember having a plurality of openings, and electrically conductiveelements forming an electrostatic shield around the space;

a plurality of dielectric members each having a peripheral edge and eachdisposed to close a respective opening; and

sealing means forming a hermetic seal between the housing member and theperipheral edge of each of the dielectric members.

BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWING

FIG. 1 is a perspective view of a first embodiment of a large areaplasma source according the present invention.

FIG. 2 is a view similar to that of FIG. 1 showing a second embodimentof the plasma source according to the invention.

FIG. 3 is a perspective, detailed view of a portion of one component ofthe embodiment shown in FIG. 1, with several elements shown in explodedform.

FIG. 4 is a cross-sectional, elevational view taken along line 4—4 ofFIG. 1.

FIG. 5 is a view similar to that of FIG. 3, showing a second embodimentof the component of the source shown in FIG. 1.

FIG. 6A is a view similar to that of FIG. 3, showing a third embodimentof the component of the source shown in FIG. 1.

FIG. 6B is a view similar to that of FIG. 6A showing a modified form ofthe embodiment of FIG. 6A.

FIG. 6C is a cross-sectional detail view of a portion of the structureshown in FIG. 6B.

FIG. 7 is a cross-sectional plan view of an embodiment of the sourceillustrated in FIG. 1, looking upwardly from within the chamber housing.

FIGS. 8A and 8B are, respectively, an elevational, cross-sectionaldetailed view and a bottom plan detailed view of components of theembodiments shown in FIG. 1.

FIG. 9 is an elevational, pictorial view illustrating the principle ofoperation of a component illustrated in FIG. 7.

FIGS. 10A and 10B are velocity distribution diagrams illustratingdifferent modes of operation of the components shown in FIG. 9.

DETAILED DESCRIPTION OF THE INVENTION

FIG. 1 illustrates a first embodiment of a processing apparatusaccording to the invention. This apparatus is composed essentially of achamber housing 2 and an enclosure 4 which surrounds housing 2. Housing2 and enclosure 4 cooperate to delimit an annular space 6 which, in thisembodiment, has a rectangular cross-section in a horizontal plane. Thisis an appropriate shape for processing substrates constituting flatpanel displays. However, other cross sections can be provided and, forsome applications, e.g., semiconductor wafer processing, a circularcross section will not only correspond to the substrate shape but alsoprovides better structural integrity than a rectangular shape.

Space 6 is filled with a liquid coolant and contains an RF coil 8 thatis supplied with an RF current which generates an electric field in theregion enclosed by housing 2 in order to ignite and sustain a plasma inprocessing region 10 enclosed by housing 2 and by upper and lower wallsof enclosure 4. Upper wall 12 of enclosure 4 carries coolant supply andreturn lines 16, vacuum pump assemblies 18 for pumping gas molecules andions out of processing region 10 and maintaining a desired vacuumpressure therein, and passages (not shown) for couplings used tointroduce fresh processing gas into region 10.

Wall 12 additionally carries a fast match assembly, or match network,20, which is a component that is known per se and that is typically madeup of an L-network of two variable capacitors and an inductor whereinthe variable capacitors are mechanically adjusted by an automaticcontrol network. The purpose of network 20 is to equilibrate the sourceimpedance of the RF generator with the load impedance as seen by thegenerator looking into the match network and plasma source. Typically,the source impedance of the RF generator is 50Ω and, hence, the variablematch network components are varied such that the output impedance ofthe match network is the complex conjugate of the input impedance to theplasma source. During matched conditions, the forward power at the matchnetwork juncture is maximized and the reflected power is minimized.Match network designs, although different in speed, robustness andcontrollability, are all based upon the same fundamental principles andare often found described in the prior art.

As will be described in greater detail below, chamber housing 2 providesthe vertical bounding walls for region 10 and is constructed towithstand the forces acting on walls 12 and 14 due to the differencebetween the atmospheric pressure acting on the outer surfaces of therelatively large area walls 12 and 14 of enclosure 4 and the vacuumpressure established in region 10 and acting on the inner surfaces ofthose walls. Chamber housing 2 is further constructed to provide anelectrostatic shield for region 10 and allow transmission of RF energyfrom coil 8 to region 10. The vertical wall of enclosure 4 may, but neednot, be constructed to assist in withstanding the above-mentioned forcesimposed on walls 12 and 14 by the pressure differentials between opposedsurfaces of each wall.

Each vacuum pump 18 is part of a vacuum pump assembly that includes arespective gate valve or throttle, 22 mounted on wall 12 with the aid ofa coupling flange 24. When gate valve 22 is left wide open, maximumpumping speed can be achieved. However, partial closure can permit aspatial variation of the pumping speed by means of varied flowrestriction through the distributed pumping orifice. Gate valve 24 canbe of a known design.

Flange 24 is a cylindrical part which provides a flow path between arespective through bore 28 in wall 12 and the inlet end of a respectivepump 18. Bores 28 are pumping ports which will each communicate with theinlet of a respective vacuum pump 18. By suitable positioning, andselection of the number, of bores 28, along with suitable selection andcontrol of the operation of pumps 18, the exhaust gas flow can be tunedfor uniform gas exit from the region 10. Selection and control of pumpsand the arrangement of bores 28 can be effected on the basis ofprinciples and practices already well known in the art.

Attainment of the desired process uniformity also requires appropriatecontrol of gas injection. This aspect of the invention will bedescribed, infra.

In addition, the bottom of the source will be provided with a suitablesubstrate support and means for applying a bias voltage, for example, anRF bias to the support. Here again, such a substrate support can beconstructed and installed in the source in accordance with principlesand practices already well known in the art.

FIG. 2 is a view similar to that of FIG. 1 showing a second embodimentof a source according to the invention which differs from the source inFIG. 1 essentially with respect to geometric configuration. Whereaschamber 10 of the embodiment shown in FIG. 1 has the form of aparallellapiped, chamber 10′ of FIG. 2 has the form of a pyramidalfrustum. Thus, the side walls of housing 2′ are inclined with respect toa vertical axis of the source as are the side walls of enclosure 4′.

FIG. 3 is a perspective, detail view showing a portion of housing 2,which is made of a conductive material, such as aluminum, and is given awall thickness sufficient to provide the requisite compressive andtensile strengths. When the interior of enclosure 4 is under a vacuum,integrated pressure forces directed toward the interior thereof willgenerate a bending moment within the material, hence creating separateregions of enclosure 4 that are under tension or compression.

Being made of a conductive material, housing 2 constitutes anelectrostatic shield. Housing 2 is provided with a series of verticallyelongated recesses 32 that are spaced at uniform intervals about thehousing periphery. At the center of each recess 32, there is a narrow,elongated slot 34 which extends through the remaining thickness ofhousing 2 to communicate with region 10. Each recess 32 is provided withan insert 36 made of a dielectric material, such as alumina, with aprojecting portion that extends into slot 34 of its respective recess32. Each insert 36 is provided with four elastomeric vacuum seals 40 and40′ at the inwardly facing surface of insert 36 and 42 and 42′ at theoutside of insert 36. Each insert 36 is covered by a respective frame 46which holds its associated insert 36 and seals 40, 42 in place in recess32, with the aid of a plurality of screws (not shown) that extendthrough screw holes 48. Between the two seals 40 and 40′ there isprovided at least one passage to atmosphere, shown at 52 in FIG. 4. Thispassage is fabricated to extend to a location outside of enclosure 4 foraccess when the system is totally assembled. Passage 52 allows for leakchecking both seals 40 and 40′. Thus, both sealing with respect to thecoolant fluid 54 between chamber 2 and wall 4 and sealing with respectto the vacuum in region 10 can be checked with one port.

RF energy can flow from coil 8 into region 10 through inserts 36 andslots 34.

FIG. 5 shows another form of construction of the embodiment of FIG. 1.In the form of construction shown in FIG. 5, housing 2 is provided withvertically elongated slots 60 that extend through the entire wallthickness of housing 2. In the illustrated embodiment, each slot 60 hasan outwardly diverging, or flaring, portion adjacent the exteriorsurface of housing 2, and a portion 64 having a surface that isperpendicular to the vertical walls of housing 2. A dielectric window 66is installed in portion 64 of each slot 60 and is secured to portion 64by a metal band 68 which is brazed to both portion 64 and the peripheraledge of dielectric window 66. Band 68 serves to compensate for thermalexpansion differences between dielectric window 66 and housing 2. FIG. 5shows a broken-away portion of the outer vertical surface of housing 2.The reason for this portion is to better illustrate the cross-sectionalform of the regions between adjacent slots 60.

Band 68 may be made of Kovar™, which is a trade-name for a metal alloycontaining 54% iron, 29% nickel and 17% cobalt. The coefficient ofthermal expansion for Kovar is between that of the metal housing anddielectric window. The use of such a material is common to the industry.

In the embodiment of FIG. 5, each slot 60 can be given a larger areathan each slot 34 of the embodiment shown in FIGS. 3 and 4, so that theFIG. 5 embodiment is capable of providing a larger total effectivedielectric window area for passage of RF energy into region 10. Inaddition, the structure, or structural area, necessary to fasten thedielectric window to the housing walls of the plasma source is minimizedin this embodiment.

FIG. 6A illustrates a further embodiment of a chamber housing 72according to the invention. Housing 72 has the same general form ashousing 2 of FIG. 1, but is provided on each side with a large areaopening 74 surrounded by a recessed portion 76 that frames opening 74.Each opening 74 is completely covered by a rigid dielectric panel 78made, for example, of alumina. Each panel 78 is a one-piece dielectricbody composed of a flat base portion 80 and a plurality of verticallyextending ribs 82 that project at right angles from base portion 80.Panel 78 is dimensioned to extend entirely across opening 74 andrecessed portion 76.

Housing 72 is further provided with a plurality of elongated loadsupporting members 86 made of electrically conducting material, such asaluminum. Each member 86 has a T-shaped cross section, is seated betweentwo adjacent ribs 82 and is securely connected at its upper and lowerends to top and bottom edges of housing 72. Members 86 function as theconductive members of the electrostatic shield and it is important thesemembers have good RF electrical contact to housing 72 both at the topand bottom. Satisfactory contact, and a sound mechanical connection, canbe provided by using machine screws (not shown) to secure the upper andlower ends of each member 86 to the top and bottom edges of housing 72.Because members 86 are made of metal and are therefore relativelyinelastic, a layer of an elastic material is preferably disposed betweeneach member 86 and its associated part of base portion 80. One suchmember 88 is shown in broken lines in FIG. 6A.

As an alternative to the embodiment illustrated in FIG. 6A, theelectrostatic shielding may be provided in the form of a metal coatingon the external surface of each panel 78, as shown in FIG. 6B. Thiscoating will be in the form of individual strips each located betweentwo adjacent ribs 82. In this embodiment, ribs 82 are shortened so thattheir ends are spaced from the upper and lower edges of each panel 78.Upper and lower window braces 89 (only upper brace 89 is shown in FIG.6B) extend along the upper and lower edges of each panel 78, areprovided with notches that receive the ends of ribs 82 and with tabswhich interlock with the ribs 82. These tabs are in direct contact withthe applied metal coating. Window braces 89 secure the entire perimeterof each dielectric panel 78 to the associated opening in housing 72.Braces 89 are bolted to housing 72 as shown in FIG. 6B.

Each brace 89 has certain built in features that are shown in FIG. 6C.In particular, FIG. 6C shows O-ring seals 40 and 40″ and leak testingport 52′. In both embodiments presented in FIGS. 6A and 6B, eachdielectric panel 78 must be capable of withstanding the bending momentsimposed due to inwardly directed pressure forces present when a chambervacuum exists. This the primary purpose of ribs 82.

In the case of the embodiment shown in FIG. 6A and the alternativethereto described above, each dielectric panel 78 is hermetically sealedto its associated recessed portion 76 by at least two O-rings held ingrooves 41. Preferably, sealing is achieved by the provision of dualelastomeric seals, like rings 40 and 40′, separated by a space which iscoupled by a series of passages, like passage 52, internal to thehousing to the outside to allow technicians to sense liquid leaks orvacuum leaks.

FIG. 7 is a cross-sectional plan view showing the wall of chamber 2 andthe bottom surface of wall 12 when one looks upwardly from withinchamber region 10. Wall 12 carries an array of processing gasintroduction tubes 90 having vertically extending inlet portions 92 thatextend through, and are supported by, dielectric inserts 96 that aresecured in openings 98 in wall 12. Inserts 96 and other dielectric partsof this assembly may be made of, for example, PTFE. Each tube 90 has ahorizontally extending outlet portion 94 that extends between itsassociated inlet portions 92. Outlet portion 94 of each tube 90 isprovided with a row of outlet holes, or injection nozzles, 94′ (FIGS. 8Aand 8B) extending along the length thereof.

Outlet portions 94 of tubes 90 can be placed at a height above thesubstrate to be treated that allows the optimum gas species to arrive atthe substrate. As the distance between outlet portions 94 and thesubstrate is increased, the spacing between tubes 90 can be increasedwhile the density of ionized gas reaching the substrate remainsapproximately uniform. Of course, an increase in the spacing betweentubes 90 results in a reduction in the number of tubes. For certainprocesses, however, it may be desirable to bring the outlet portions oftubes 90 closer to the substrate. This may be done, for example, if itis desired to reduce the time between gas ionization and contact of theresulting ions with the substrate.

Each gas injection tube input portion is connected to a flow regulatingvalve or an individual mass flow controller to control the injection ofgas from an inlet manifold (not shown). By controlling the flow of gasinto each end of each tube 90, a variety of gas injection profiles aremade possible, as will be described below.

At the center of wall 12 there is provided a viewport 99 in the form ofa funnel-shaped passage. This funnel-shaped passage is angled such thatit provides a field of view encompassing the entire substrate beingprocessed. Viewport 99 may be used simply for visual inspection of thechamber and its process, or it may accommodate a diagnostic systemrequiring optical access to the inner chamber.

The exterior surfaces of gas introduction tubes 90 will become coatedwith residue of the processing gasses over the course of time. Accordingto a further feature of the invention, these coatings can be removedfrom tubes 90 by applying RF bias to tubes 90 during cleaning of theinterior of housing 2.

Such cleaning is conventionally performed periodically in present dayetch or deposition chambers by a separate cleaning process wherein thechamber is cleaned with a substrate installed in region 10. Region 10 isfilled with a gas which, when ionized in a plasma, is capable ofremoving residue coating from surfaces within housing 2 and a plasmagenerating RF field is created in region 10. In wafer processing, thiscleaning process is often conducted at very much higher pressure thanthe normal process pressure to improve the chemical process rate byincreasing the number of atoms, or ions, in the plasma. Applying RF biasto parts of housing 2 can also increase the residue removal rate.

It is also known to install a metal electrode outside the housing andbehind the dielectric wall of the housing and to apply a voltage to theelectrode in order to provide bias to the wall and increase the cleaningrate. An arrangement of this type is disclosed, for example, in pendingprovisional application No. 60/065,794, by Wayne Johnson, entitled ALLRF BIASABLE AND/OR SURFACE TEMPERATURE CONTROLLED ESRF.

Application of RF bias to tubes 90 can increase the energy of ionbombardment within tubes 90 and thus increase the rate and effectivenesswith which residues are etched away from their interior walls. Ionbombardment can be thought of as increasing the surface temperature ofsurfaces being bombarded and hence can increase the chemical reactionrates.

Preferably, injection tubes 90 are made of anodized aluminum or arecomposed of a metal tube component sheathed in dielectric tubing made ofquartz or alumina.

To allow for application of a cleaning RF bias voltage to tubes 90, itis necessary to isolate tubes 90 electrically from the walls ofenclosure 4. Such isolation is needed so that plasma is not createdinside the tubes that deliver gas. This bias will not be applied duringnormal operation, but only during periodic cleaning cycles. The RF biasto the gas injection tubes is applied periodically to clean the exteriorsurface (or process side). During etch process, contaminants may buildup on the tube surface. To minimize long-term particulate contamination,the exterior surface of the injection tubes (in addition to all surfaceswithin the chamber) must be cleaned during a cleaning cycle. The RF biaswill generate a DC self-bias (and resultant average voltage differenceacross the sheath) which in-turn affects the average ion energydelivered to the injection tube surface.

If the tubes are made of conductive material, then a capacitor is neededto allow them to charge by self-bias.

In order to prevent the generation of a plasma within the interiorvolume of the injection tubes during RF bias application, it is possibleto use strictures inside the tube that minimize breakdown by use ofdielectric surface area. One example is a bundle of capillary tubes ofdielectric material (quartz) in the gas flow path as has been used inthe semiconductor industry to deliver process gas to an upperelectrode-inject plate.

FIGS. 8A and 8B show details of an arrangement for supplying RF voltageto an injection tube 90. FIG. 8A is a cross-sectional view and FIG. 8Bis a bottom plan view of the entrance region of one injection tube 90via which process gas is supplied and RF bias is applied. This assemblyresides within upper wall 12. Process gas is fed from a standard gasline and fitted to the gas injection system using a standard fitting102, as shown. Dielectric inserts 96 isolate fitting 102 from aconductive base ring 104 which surrounds inlet portion 92. A RF voltageis applied to ring 104 through a standard connection flange made at theoutput of the respective match network, an RF connection interface 108and a RF inner conductor 110 which forms a unit with base ring 104. TheRF feed input to the gas injection RF bias assembly is a standard feedconsisting of inner conductor 110, an outer conductor 114 and adielectric 116 sandwiched between the conductors. Inner conductor 110attaches to base ring 104 that is in immediate contact with gasinjection tube 90. Outer conductor 114 is integral with a support plate120. Dielectric inserts 96 and 116 isolate outer conductor 114 and itssupport plate 120 from inner conductor 110 and ring 104.

According to the invention, the distribution of processing gas withinregion 10, using the injection tube arrangement shown in FIG. 7, can becontrolled by suitable selection of one or more of gas inlet pressure,gas flow rate and the total area of the injection nozzles (94′) in eachtube 90. The relationships involved in effecting this control will bedescribed with reference to FIGS. 7, 9, 10A and 10B.

As shown in FIG. 7, processing gas enters region 10 via the array ofinjection tubes 90 whose outlet portions 94 lie in a common horizontalplane located at a selected vertical distance below enclosure wall 12,which wall constitutes a pump manifold plate. However, it is notnecessary that all injection tubes 90 lie in a common horizontal plane.In fact, it may be beneficial to vary their vertical spacing relative tothe wafer plane.

Injection tubes 90 are equally spaced (with spacing d) in one horizontaldirection. As illustrated in FIG. 9, outlet portion 94 of each tube 90has a length 2 L in a horizontal direction across region 10. As notedearlier herein, the number of tubes 90, and their spacing, d, can bevaried to achieve a selected processing result. Equally, length 2 L isselected on the basis of the desired processing result. Each injectiontube 90 has a cross-sectional area A₁ along its entire length, i.e.,along its inlet and outlet portions, and outlet portion 94 of each tube90 is provided with N injection nozzles (94′ in FIGS. 8A and 8B) eachhaving a cross-sectional area A₂. Thus, the injection nozzles in onetube 90 have a total outlet area A_(2T)=NA₂. The injection nozzles areuniformly spaced apart with a spacing Δl between the center lines ofadjacent injection nozzles. Gas is fed to both ends of a tube 90 with aninlet pressure P_(t) and a volume flow rate Q at each end, i.e., at eachinlet portion. This is also elaborated in the discussion of FIG. 9. Thecross-sectional area of each tube 90 need not be constant nor do theinjection nozzles need to be equally spaced. Clustering of the injectionnozzles may be beneficial for additional gas injection control.

The injection system is designed to introduce the processing gas to alarge volume chamber region 10 in which a vacuum pressure P_(c) ismaintained. The gas is introduced at subsonic speeds, i.e.,M=v/a<0.3-0.5 where M is the Mach number, v is the gas velocity at eachexit orifice and a is the local speed of sound.

According to the invention, the distribution, or gradient, of gas exitvelocities across the length of output portion 94 of a tube 90 can becontrolled by proper selection of the flow rate Q and the inlet totalpressure P_(t) at both ends of that tube 90. The particular gradientestablished will influence the uniformity of the plasma-assistedtreatment across the substrate surface.

When the flow rate Q into an injection tube 90 is high, an exit velocitydistribution can be obtained such that the velocity is greatest at themid-point of the length of the outlet portion of the tube and decreasesprogressively toward the ends thereof. Alternatively, when the flow rateQ is low, the velocity distribution is such that the greatest velocitiesare achieved at the ends of tube outlet portion 94. Hence, it ispossible to control the velocity distribution along the length of theoutlet portion of a tube 90, or the span-wise velocity distribution, byadjusting the inlet volume flow rate Q.

For the sake of clarity, the terms “high flow rate” and “low flow rate”will be defined. A “high flow rate” is one in which the gas momentum islarge relative to the difference in pressure between the gas in theinjection tube and the chamber pressure. Similarly, a “low flow rate” isone in which the relative gas momentum is small. In the case of a highflow rate, the lateral pressure gradient is unable to sufficiently bendthe “high” momentum fluid and accelerate it through the adjacentinjection nozzle; therefore, the predominant mechanism is decelerationof the gas to a stagnation pressure at the midpoint of the length of theoutlet portion 94 of the tube 90 where momentum is cancelled and asufficient pressure difference can be achieved. The stagnation flow atthe outlet portion midpoint is simply a consequence of introducingprocess gas at both ends of the injection tube. In the case of a lowflow rate, the gas momentum is such that gas will tend to exit viaopenings at the ends of the outlet portion of the injection tube underthe effect of the differential between the inlet pressure and thepressure in chamber region 10; as gas exits through successive injectionnozzles 94′, the pressure within outlet portion 94 of a tube 90decreases.

Taking a more rigorous approach, the above explanation can besubstantiated in a clearer manner. Consider the transverse equation ofmomentum for the coordinate system indicated in FIG. 9. Assuming theflow to be steady and two-dimensional and neglecting the viscous terms,the transverse equation of momentum becomes: $\begin{matrix}{{\frac{- 1}{\rho}\quad \frac{\partial P}{\partial r}} = {{v_{z}\quad \frac{\partial v_{r}}{\partial z}} + \frac{\partial v_{r}}{v_{r}{\partial r}}}} & (1)\end{matrix}$

The radial pressure gradient is balanced by two terms; the first ofwhich represents the transfer of stream-wise momentum (in the directionz) into radial momentum (in the direction r), and the second of whichrepresents the radial acceleration of the radial flow. The injectiontube design depends on an independent set of parameters including ρ₀, Q,P_(t), P_(c), A₁, A_(2t), Δl and L; refer to FIG. 9. Note that thisparameter list excludes the number N of injection nozzles 94′ and theirrespective cross-sectional areas A₂ since N=2 L/Δl and A₂=A_(2T)/N).Neglecting compressibility effects (a good assumption for M<0.3), theradial equation of momentum is non-dimensionalized using the followingrelationships $\begin{matrix}{z^{*} = \frac{z}{\Delta \quad l}} & \text{(2a)} \\{r^{*} = \frac{A_{2T}r}{2A_{1}\Delta \quad l}} & \text{(2b)} \\{v_{z}^{*} = \frac{A_{1}v_{z}}{Q}} & \text{(2c)} \\{v_{r}^{*} = \frac{A_{2T}v_{r}}{2Q}} & \text{(2d)} \\{P^{*} = \frac{P}{\Delta \quad P}} & \text{(2e)} \\{\rho^{*} = \frac{\rho}{\rho_{0}}} & \text{(2f)}\end{matrix}$

where ΔP=P_(t)−P_(c); ρ represents local density, and ρ₀ representsdensity at stagnation conditions. The radial length and velocity scalesare obtained by virtue of continuity. Hence, the non dimensional radialequation of momentum is obtained, $\begin{matrix}{\frac{\partial P^{*}}{\rho^{*}{\partial r^{*}}} = {B^{*}\left( {{v_{z}^{*}\quad \frac{\partial v_{r}^{*}}{\partial z^{*}}} + {v_{r}^{*}\quad \frac{\partial v_{r}^{*}}{\partial r^{*}}}} \right)}} & (3)\end{matrix}$

which identifies the non-dimensional parameter $\begin{matrix}{B^{*} = \frac{4\quad \rho_{0}Q^{2}}{A_{2T}^{2}\Delta \quad P}} & (4)\end{matrix}$

When B*>>1, this corresponds to high flow rates at which the pressuregradient is inadequate to substantially turn the stream-wise momentumand, hence, larger velocities exit at the midpoint, or center, of theoutlet portion of the injection tube. Conversely, B*<<1 corresponds tolow flow rates at which the opposite is true. The resulting velocitydistributions are depicted in FIGS. 10A and 10B.

Thus, if B*=1, the gas exit velocity will be constant along the lengthof outlet portion 94 of tube 90. For many processes, this will be thepreferred exit velocity distribution. However, there may be situationsin which it is preferable that B*≠1. For example, the RF field generatedin region 10 may vary in intensity in radial directions perpendicular tothe vertical center axis of region 10. In such a case, a gas flow ratevariation having a form shown in one of FIGS. 10A and 10B may be used tocompensate for the RF field variation in order to produce a processingresult which is uniform across the surface of the substrate.

Close inspection of the definition of B* provides insight into thedesign of injection tubes 90. For example, the condition B*>>1 can beachieved by performing any one of the following actions while holdingall other parameters constant: increase Q (increase the gas momentumρ₀V); decrease ΔP (reduce the turning force); and decrease A_(2T)(provide greater flow resistance).

Typically, a fixed relation will exist between a given value for P_(c),inlet pressure and flow rate. However, it might be possible toindependently control the inlet total pressure and the mass flow rate.This would require adjusting the total pressure losses in the systemusing throttle valves. For example, the throttle valve upstream of theturbo-molecular pump can adjust the chamber pressure and pressureregulators upstream of the injection tubes can regulate the totalpressure.

Considering the list of independent dimensional parameters listedpreviously, it is sufficient to define a parameter for uniformityu=P(z=0)−P(z=L) such that the non-dimensional uniformity u*=u/ΔP takesthe form $\begin{matrix}{u^{*} = {u^{*}\left( {B^{*},\frac{\Delta \quad l}{L},\frac{\Delta \quad P}{P_{c}},\frac{A_{2}}{A_{1}},\frac{A_{1}}{L^{2}}} \right)}} & (5)\end{matrix}$

We consider the asymptotic limit where the four latter parameters go tozero, i.e., the number of injection nozzles is large (Δl/L→0), thepressure difference is small relative to the absolute value(ΔP/P_(c)→0), each injection nozzle area A₂ is small relative to theinjection tube cross-sectional area (A₂/A₁→0), and the injection tube islong relative to its diameter (A₁/L²→0). Nominal conditions for B*˜1are: Δl=1.0 cm, L=50 cm, N=100, A₁=1.77 cm², A₂=0.0079 cm², P_(c)=500mTorr, P_(t)=600 mTorr and Q=160 sccm (or Q_(tot)=320 sccm).

According to various alternatives made possible by the invention, it maybe desirable to allow the gas flow to choke at the gas injection nozzles94′. When the pressure ratio across an injection nozzles 94′ (i.e., theratio of the total pressure inside the injection tube to the ambientchamber pressure beyond the exit of the injection nozzle) issufficiently large, the injection nozzle reaches a “choked” conditionwherein the volume flow rate is invariant with either further reductionof the back pressure (or chamber pressure) or increase of the inlettotal pressure. In fact, the mass flow can only be increased further byincreasing the total inlet pressure (hence affecting the gas density).Since the volume flow rate at an injection nozzle exit is invariant andthe injection nozzle exit area is constant, this implies that the exitvelocity is constant. However, one may redistribute the injectionnozzles 94′ in injection tube 90 in order to affect the mass flowdistribution entering the chamber. Hence, the mass flow distribution maybe designed to behave in either manner as described in FIGS. 10A and10B. For example, if injection nozzles 94′ are clustered towards theends of injection tubes 90, then a mass flow distribution similar toFIG. 10B can be obtained. Conversely, if injection nozzles 94′ areclustered towards the center of the injection tubes, then a mass flowdistribution similar to FIG. 10A can be obtained. Additionally, far fromthe injection plane (approximately 10 to 20 injection nozzle diameters)the velocity distribution will behave similar to these distributionforms.

There are several advantages to the use of the gas injection tubes,namely: RF bias of the gas injection tubes allows periodic cleaning ofthe exterior surface; variable placement of adjacent tubes in separatevertical and/or horizontal planes, allowing each injection tube to belocated at a different vertical height above the substrate for improvedprocess control; selectable injection nozzle distribution formodification of inlet mass flow distribution; supersonic or subsonicinjection capability; and adjustment of gas momentum in the injectiontubes for mass flow redistribution subsonic injection.

As mentioned earlier herein with reference to FIGS. 1 and 2, a plasmasource according to the invention includes a plurality of pumps 18 thatcontinuously withdraw gas from region 10 during a processing operation.Each pump 18 acts primarily on an associated section of region 10.Processing at high rates requires high gas throughput to get the desirednumber of surface chemical reactions to occur. Processing needs high gasthroughput and high plasma density. The large number of pumps employedin the embodiments disclosed herein provide the high pumping capacityneeded to achieve high processing rates for large area substrates. Eachpump is fitted with a throttle control valve, as shown at 22 in FIG. 1,that can regulate the pumping speed of that pump. By individuallycontrolling the pumping speed of each pump 18, a wide variety of pumpingspeed profiles are possible. Each pump 18 may be constituted by any typeof pump currently employed in plasma processing apparatus. Solely by wayof non-limiting example, each pump 18 may be a turbomolecular pump, orturbo-pump, with or without a backing pump.

As indicated in FIGS. 1 and 2, apparatus according to the invention maybe equipped with sixteen (4×4) pumps 18. Each pump may be a 1000liter/sec turbo-pump positioned atop wall 12. Processing gas is directedtowards the substrate from injection nozzles 94′ in injection tubes 90by the momentum created by the velocity at which the processing gasexits from the injection nozzles. After interacting with the substrate,unused process gas and volatile reaction products are removed via pumps18. In order to minimize the interaction of reaction products withincoming process gas, an outward pressure gradient is established tobias the reaction products to flow to the outer walls and then upward tothe pumps. This is accomplished by reducing the pumping rate of thepumps 18 that are proximate to the center of plate 12, for example byslightly closing the valves of the those pumps to decrease their pumpconductance. In this manner lower chamber pressures can be achievedtowards the walls of the chamber housing 2, 2′.

While the description above refers to particular embodiments of thepresent invention, it will be understood that many modifications may bemade without departing from the spirit thereof. The accompanying claimsare intended to cover such modifications as would fall within the truescope and spirit of the present invention.

A plasma source according to the invention may include a conventionalchuck for holding the substrate, or wafer, to be processed. The chuckwould be typical of most conventional plasma sources. In addition toholding the substrate, it should be capable of applying a RF bias to andheating the substrate. Therefore, for large area processing, this chuckcould consist of multiple segments.

The presently disclosed embodiments are therefore to be considered inall respects as illustrative and not restrictive, the scope of theinvention being indicated by the appended claims, rather than theforegoing description, and all changes which come within the meaning andrange of equivalency of the claims are therefore intended to be embracedtherein.

What is claimed is:
 1. A chamber housing enclosing a plasma region in alarge area plasma source used for performing plasma assisted processeson large area substrates, said chamber housing comprising: a housingmember constituting a substantially vertically extending wallsurrounding a space corresponding to the plasma region, said housingmember having a plurality of openings, and electrically conductiveelements forming an electrostatic shield around the space; a pluralityof dielectric members each having a peripheral edge and each disposed toclose a respective opening; and sealing means forming a hermetic sealbetween said housing member and said peripheral edge of each of saiddielectric members.
 2. The chamber housing according to claim 1 whereinsaid housing member has a plurality of recesses, each of said openingsis formed in a respective recess, and said sealing means are disposedbetween said peripheral edge of each of said dielectric members and saidrecesses.
 3. The chamber housing according to claim 2 wherein said wallhas a polygonal form with a plurality of flat sides.
 4. The chamberhousing according to claim 3 wherein each of said sides has a singleopening.
 5. The chamber housing according to claim 4 wherein saidelectrically conductive elements are metal bars extending between, andsecured to, upper and lower edges of said wall, each of said metal barsextending across a respective opening.
 6. The chamber housing accordingto claim 5 further comprising elastic members interposed between saidmetal bars and said dielectric members.
 7. The chamber housing accordingto claim 3 wherein each of said sides has a plurality of openings. 8.The chamber housing according to claim 7 wherein said electricallyconductive elements are constituted by portions of said wall that extendbetween top and bottom edges of said wall and are interposed betweensaid openings.
 9. The chamber housing according to claim 8 wherein eachof said dielectric members has a projecting portion that extends intosaid opening.
 10. The chamber housing according to claim 3 wherein saidflat sides extend vertically.
 11. The chamber housing according to claim3 wherein said wall has the form of a pyramidal frustum.
 12. The chamberhousing according to claim 2 wherein: said sealing means comprise, foreach of said dielectric members, two annular seats disposed at aninterface between said dielectric member and its associated recess, saidseals being spaced apart along the interface; said wall has a passagehaving a first end in communication with the interface and a second endremote from the interface; and said housing further comprises pressuremonitoring means connected to said second end of said passage to monitorleakage of fluid past either one of said seals.
 13. A large area plasmasource used for performing plasma assisted processes on large areasubstrates in a plasma region, said source comprising: the chamberhousing according to claim 1; a coil surrounding said housing andoperative for generating an RF field in the plasma region; an enclosuremember surrounding said housing and the plasma region; gas injectionmeans extending through said enclosure member for introducing anionizable processing gas into the plasma region; substrate support meansfor supporting a substrate to be processed in the plasma region; and aleast one pump disposed for pumping gas out of the plasma region tomaintain a low pressure in the plasma region.
 14. The plasma sourceaccording to claim 13 wherein said gas injection means comprise at leastone gas injection tube having at least one inlet portion for receiving asupply of the ionizable processing gas and an outlet portion providedwith a plurality of injection nozzles via which the ionizable processinggas is conveyed from said inlet portion to the plasma region, whereinsaid outlet portion extends parallel to a substrate supported on saidsubstrate support means.
 15. The plasma source according to claim 14wherein said at least one gas injection tube has two inlet portions andsaid outlet portion is interposed between said two inlet portions. 16.The plasma source according to claim 15 wherein said outlet portion ofsaid at least one gas injection tube extends in a straight line betweensaid two inlet portions.
 17. The plasma source according to claim 16wherein said at least one gas injection tube comprises a plurality ofgas injection tubes spaced from one another in a plane parallel to asubstrate supported on said substrate support means.